Metallic materials based on martensitic steel

ABSTRACT

The material is constituted by a regular alternation of relatively thick layers of martensitic structure and thinner layers of austenitic structure. The product is useful for sheet metal exposed to the action of hydrogen.

United States Patent [191 Montuelle et al.

111 3,849,079 Nov, 19, 1 974- METALLIC MATERIALS BASED ON 2,25 8,56410/1941 Armstrong et a1. 138/ 142 MARTENSXTIC STEEL 2,472,320 6/1949Vennerholm et al. 148/ 16.5 2,544,335 3/1951 Linnert 29/191 [75]Inventors: Jean Montuelle, Bourg-la-Reine; 2,921,377 1/1960 Samuel eta1. Georges Chaudron, Paris; Gerard 3,071,981 1/1963 KuntzmannPiaard-Legry, Fontenay-aux-Roses, 3,073,015 1/1963 Machtell et a1 all ofFrance 3,093,556 6/1963 Machu et a1 3,148,954 9/1964 H [73] Assignee:Agence Nationale de Valorisation de 3132,1353 1/1966 C221 la Recherche(Anvar), Paris, France 3,240,572 3/1966 Rieden 3,325,259 6/1967 Mayer eta1 [22] 1970 3,343,928 9/1967 Bellis et al. 21 App} 9 237 3,357,86812/1967 Tanczyn 3,359,083 12/1967 Leichter 3,511,283 5/1970 Iannone138/143 [30] Foreign Application Priority Data Dec. 19, 1969 France69.44094 Primary ExaminerWinston A. Douglas Assistant Examiner-O. F.Crutchfield [52] 11.8. C1. 29/183.5 Attorney, Agent, or Firm-Larson,Taylor & Hinds [51] Int. Cl. C23c 39/20 [58] Field 0188811211....148/34, 39; 57 ABSTRACT The material is constituted by a regularalternation of 56] References Cited relatively thick layers ofmartensitic structure and UNITED STATES PATENTS thinner layers ofaustemtic structure. The product is useful for sheet metal exposed tothe action of hydro- 2,048,276 7/1936 Marlies et a1. 29/195 A gen.2,133,293 10/1938 Gordon 29/187.5 X 2,226,403 12/1940 Hopkins 148/34 X10 Claims, 3 Drawing; Figures P\\\ \\\\\\\\r\\\\\\ W if {1 K14.

W y ////y k\\ A METALLIC MATERIALS BASED ON MARTENSITIC STEEL Theinvention relates to metallic materials based on martensitic steel,i.e., metallic materials constituted of a major portion of a steel ofmartensitic structure, the expression metallic material being taken herein a very general sense and including both semimanufactured products(sheet metal for example) and more fabricated products which havereached their final form from the industrial point of view (bars ortubes e.g.).

The invention relates also to methods of manufacture of such metallicmaterials based on martensitic steel.

Before introducing the main feature of the invention and to enable theoriginality and advantage of this feature to bebetter appreciated, itwould be opportune to briefly recall here certain concepts, well knownto metallurgists, relating to steels with martensitic structures and tostainlesssteels with austenitic structure.

Steels with martensitic structure, especially maraging" steels(generally containing nickel, cobalt and molybdenum, but no chromium),have high mechanical properties (breaking load capable of reaching 230hec tobars), but, on the other hand, they are subject to corrosionphenomena by external agents and, especially, to the deleteriousphenomenon of fragilization by hydrogen.

On the other hand, stainless steels of chrome-nickel, even materialsbased on a nickel, have, in the austenitic phase, an excellentresistance to corrosion but properties distinctly less, from themechanical point of view, than the steels of martensitic structure.

There will be appreciated, under these conditions, the advantage whichcould be offered by a metallic material reconciling the properties(recounted above) of of fundamental researches which have shown that amaterial constituted of a steel of martensitic structure (especiallymaraging steel) could, whilst preserving its excellent mechanicalqualities, be effectively protected, against corrosion and the risks offragilization by hydrogen, by a superficial coating constituted by alayer of austenitic structure obtained by diffusion of chromium, thesaid layer playing the role of a diffusion barrier vis-a-vis theattacking hydrogen agent coming from the outside.

For convenience in the description, such a protective layer ofaustenitic structure will be denoted below by the expression austeniticlayer, whether it relates to a stainless steel (especiallychrome-nickel) or to a material based on nickel, even pure nickel.

The metallic material according to the invention is 'a massive ferrousmaterial constituted by a regular alteraverage thickness of the adjacentaustenitic layers situated respectively on both sides of the martensiticlayer concerned,'the massive metallic material thus constituted having,due to its mixed austeno-martensitic stratified structure withpredominance of martensite by weight, on the one hand, high mechanicalproperties (breaking load greater than hectobars) due to thepredominance of the martensitic phase hardenable by intermetallicprecipitation, on the other hand, an excellent resistance to corrosionand especially to fragilization by hydrogen, this resistance in depth ofthe material resulting from the presence of successive diffusionbarriers constituted by the austenitic layers, and, on the other handlastly, the qualities inherent in stratified materials with rigid layers(martensitic layers) separated by ductile layers (austenitic layers. ofwhich the ductility is approximately five times greater than that of themartensitic layers).

The martensitic layers can all have a same thickness and the austeniticlayers also a same thickness (less than the preceding one).

But it would also be possible to give various thicknesses to certain atleast of the martensitic layers and- /or to certain at least of theaustenitic layers, with the reservation only of preserving thepreponderance to the martensitic layers.

Thus, for example, it would be advantageous to increase the size of theaustenitic layers, playing the role of diffusion barriers, on the sideof the face most exposed to the manufactured element if the said elementhas in effect a face more exposed than the other to attacking agents.

In any case, in the majority of cases, it would be convenient toattribute to each martensitic layer, as already indicated, a thicknessequal to at least five times, and preferably of the order of ten times,the average thickness of the two austenitic layers situated on bothsides of the martensitic layer concerned.

As for the method according to the invention, it consists, with a viewto obtaining the metallic material with alternated martensitic layersand austenitic layers which has just been considered,

in manufacturing first, in a first operation conducted at a temperaturecomprised between 850 and 1000C and preferably of the order of 900 to950C, a material with alternated layers of austenitic structure and ofnon-hardened martensitic structure, the latter layers being preponderantfrom the weight point of view,

and in then subjecting, the material resulting from this firstoperation, to a thermal hardening treatment of the martensitic layers,this hardening treatment being preferably of the type of conventionalhardening methods for maraging steels (e.g. a treatment for three hoursat 480C).

The invention will, in any case, be better understood with the aid ofthe supplementary description which follows, as well of the accompanyingdrawing, which supplement and drawing are given purely by way ofillustrative but non-limiting example of the methods of application andof the preferred embodiments.

FIG. 1 shows, in section with parts removed and with a considerableenlargement, a stratified composite sheet metal according to theinvention.

FIGS. 2 and 3 show, under the same conditions as FIG. 1, a stratifiedcomposite bar and tube according to the invention.

The compositesheet metal illustrated in H0. 1 is formed ofalternatedlayers of martensite M and austenite A. All the layers 'M havea same thickness E and all the layers A a same thickness e at least fivetimes less than E and, preferably, of the order of one tenth of E.

ble to manufacture such a composite sheet metal,

by welding edge to edge, e.g., in an oven with electronic bombardement,the constituent elements of an alternated stack of thin sheets ofstainless steel containing nickel (even thin sheets of nickel) and ofthicker sheet metal of maraging steel,

and then proceeding, at a temperature of 900 to 950C, with a co-rollingof the stratified blank thus obtained, in order to obtain anchoring ofthe said leaves and sheets through their surfaces in contact, in thecourse of hot colaminating, the ratio of thicknesses of the two types oflayers remaining constant,

and making the composite sheet metal thus obtained undergo a hardeningtreatment of the martensitic layers, e.g. by heating for three hours at480C.

According to a variation, a composite sheet metal according to theinvention can be obtained from a simple stack of sheets of maragingsteel having a layer of nickel on their two faces.

In any case, the outer surfaces of the sheet metal are preferably formedby austenitic layers, i.e., by the lay-- ers more adapted to resistingcorrosion.

FIG. 2 shows a bar or rod formed of alternated coaxial layers ofmartensite M and austenite A, the martensite being predominant from theweight point of view and the respective thicknesses E and e of thelayers M and A being advantageously able to satisfy the dimensionalcriteria explained previously, in a general form, relative to theabovesaid thicknesses.

Such a bar could be used for the reinforcement of concrete and, whateverits application, it would be advantageous to make it include anaustenitic outer layer Al in order to protect it against corrosion byenvironmental agents.

FIG. 3 finally, shows a tube formed of alternated coaxial layers ofmartensite M and of austenite A, the respective thicknesses E and e ofthe said layers being able, this time again, to satisfy the abovesaiddimensional criteria.

Such tube could be suitable for channeling gas or destructive liquidsunder pressure and it would then be advantageous to make it include aninner wall A0 of austenitic structure, its outer wall Al being also ableto be austenitic, especially if there is risk of destruction from theoutside.

In a general way, the fields of application of the composite elementsaccording to the invention are multiple and there can be mentioned,especially, the fields of aerospace technology and of chemical andnuclear engineering.

We claim:

1. Massive ferrous metallic material having high me chanical strengthand resistance in depth of the material to hydrogen embrittlementconsisting essentially of a regular alternation of a plurality of layersof martensitic steel and of layers of austenitic structure having aresistance to corrosion higher and a mechanical resistance lower thanthose of said martensitic steel, the thickness of each martensitic layerbeing of a magnitude equal to at least five times the average thicknessof the adjacent austenitic layers situated respectively on both sides ofthe martensitic layer concerned.

2. Massive ferrous metallic material according to claim 1 wherein theouter surface thereof comprises a layer of said austenitic structure.

3. Metallic material according to claim 1, wherein said magnitude isabout ten times.

4. Metallic material according to claim 1, wherein each of themartensitic layers have the same thickness and .each of the austeniticlayers have the same thickness.

5. Metallic material according to claim 1, wherein the martensiticlayers have different thicknesses, and the austenitic layers havedifferent thicknesses.

6. Metallic material according to claim 1, wherein said material is ametal sheet with parallel layers.

7. Metallic material according to claim 1, wherein said material is abar with coaxial layers.

8. Metallic material according to claim 1, wherein said material is atube with coaxial layers.

9. Metallic material according to claim 1, wherein the thickness of themartensitic layers is between 1000 and 50 microns and the thickness ofthe austenitic layers is between and 10 microns.

10. Iron base material according to claim 1, wherein said layers ofaustenitic structure are of a material selected from the groupconsisting of Cr. Ni. austenitic steels, nickel and nickel-base alloys.

1. MASSIVE FERROUS METALLIC MATERIAL HAVING HIGH MECHANICAL STRENGTH ANDRESISTANCE IN DEPTH OF THE MATERIAL TO HYDROGEN EMBRITTLEMENT CONSISTINGESSENTIALLY OF A REGULAR ALTERNATION OF A PLURALITY OF LAYERS OFMARTENSITIC STEEL AND OF LAYERS AUSTENITIC STRUCTURE HAVING A RESISTANCETO CORROSION HIGHER AND A MECHANICAL RESISTANCE LOWER THAN THOSE OF SAIDMARTENSIC
 2. Massive ferrous metallic material according to claim 1wherein the outer surface thereof comprises a layer of said austeniticStructure.
 3. Metallic material according to claim 1, wherein saidmagnitude is about ten times.
 4. Metallic material according to claim 1,wherein each of the martensitic layers have the same thickness and eachof the austenitic layers have the same thickness.
 5. Metallic materialaccording to claim 1, wherein the martensitic layers have differentthicknesses, and the austenitic layers have different thicknesses. 6.Metallic material according to claim 1, wherein said material is a metalsheet with parallel layers.
 7. Metallic material according to claim 1,wherein said material is a bar with coaxial layers.
 8. Metallic materialaccording to claim 1, wherein said material is a tube with coaxiallayers.
 9. Metallic material according to claim 1, wherein the thicknessof the martensitic layers is between 1000 and 50 microns and thethickness of the austenitic layers is between 100 and 10 microns. 10.Iron base material according to claim 1, wherein said layers ofaustenitic structure are of a material selected from the groupconsisting of Cr. Ni. austenitic steels, nickel and nickel-base alloys.